Demand for energy-intensive air travel was cited as among the major contributors to carbon emissions over the five-year period between 2009 and 2013, researchers reported in the Nature Climate Change.

Eight percent of the world’s greenhouse gas emissions was contributed by domestic and international tourism, four times more than was previously estimated, according to a new study.

Demand for energy-intensive air travel was cited as among the major contributors to carbon emissions over the five-year period between 2009 and 2013, researchers reported in the Nature Climate Change.

“Tourism is set to grow faster than many other economic sectors," with revenue projected to swell by four percent annually through 2025, noted lead-author Arunima Malik, a researcher at The University of Sydney's business school.

As in decades past, the United States is the single largest emitter of tourism-related carbon emissions, with other wealthy nations -- Germany, Canada and Britain -- also in the top ten, AFP reported.

But emerging economies with burgeoning middle classes have moved up the ranking, with China in second place and India, Mexico and Brazil 4th, 5th and 6th, respectively.

The aviation industry, in particular, accounts for nearly two percent of all human-generated C02 emissions, and would rank 12th if it were a country.

International travel involving long-haul flights is among the fastest growing sectors. This is amid an expected doubling of the total number of air passengers to 7.8 billion per year by 2036, according to the International Air Transport Association (IATA).

International travel accounts for a quarter of tourism-related carbon emissions.

For instance, a carbon pollution abatement that covered environmental costs would require an additional US$425 to offset emissions from a round-trip flight from Sydney to London, the authors calculated.